am7xxx-play: fix runtime warnings from ffmpeg

ffmpeg emits warnings at runtime about some AVFrame fields not being
set:

  [mjpeg @ 0x669040] AVFrame.format is not set
  [mjpeg @ 0x669040] AVFrame.width or height is not set

This happens in a loop so the message is repeated over and over and
becomes annoying, fix this by setting the missing values.